In drop-on-demand ink jet printers single drops are ejected from an orifice by on demand actuation of a transducer, most commonly either an electro mechanical transducer (as used in piezo-electric printers) or an electro-thermal transducer (as used in bubble jet printers). In both of these approaches, it is necessary to reliably contain the ink and refill the drop ejection region after an ink drop has been ejected through the orifice. Refill ink can be supplied from a reservoir integral with the print head (in "print/cartridge embodiments") or, via an umbilical conduit, from a remote reservoir. Usually, a combination of capillary ink feed and atmospheric pressure, transmitted hydraulically, is employed in refilling the drop ejection region. In some prior art systems, further means, e.g. positive pressure sources or gravitational forces acting via a hydrostatic head are used instead of, or to supplement, the usual approach. However, such further means are cumbersome, particularly in print/cartridge embodiments, where the desire is for compact insertable units. Thus, supplying refill ink via capillary action and transmitted atmospheric pressure is most attractive; U.S. Pat. Nos. 4,095,237 and 4,329,698 provide examples of drop-on-demand ink jet print/cartridges using this approach.
As described in U.S. Pat. Nos. 4,509,062 land 4,630,758, it is often desirable that the ink supply region be subject to a slight, relatively constant, negative pressure (or back pressure). Such back pressure prevents ink from drooling, or being easily shaken, from the orifices, but it should not be so large as to prevent adequate ink refill of the drop ejection zone. To accomplish a controlled back pressure the '062 patent suggests a bladder reservoir which exhibits a substantially constant spring force while collapsing. The '758 patent suggests filling the reservoir with a foam like material that exerts a controlled capillary back pressure. U.S. Pat. Nos. 3,967,286; 4,095,237 and 4,771,295 also point out the desirability of using a capillary foam material within ink reservoir to prevent introduction of air into the ink path to the print head.
However, there have been certain disadvantages to these prior art approaches. For example, as noted in U.S. Pat. No. 4,794,409, the resilient bladder and foam reservoir approaches have not utilized the reservoir storage volume efficiently (from the viewpoint of maximizing the percentage of the reservoir's interior volume that is available as printable ink). Also, it is noted that the back pressure is difficult to maintain constant with both prior art techniques and that the specially prepared, "cut and cleaned" foam of the '295 device adds significant cost.